EFFECT OF DIFFUSE IRRADIANCE ON THE REFLECTANCE FACTOR OF REFERENCE PANELS UNDER FIELD CONDITIONS

Reference panels for field reflectance factor measurements are not perfect lambertian diffusers, thus it is necessary to correct for this effect. The usual procedure consists of introducing a directional correction depending on the sun zenith angle without considering the diffuse irradiance. In this article, the effect of diffuse irradiance on the apparent reflectance factor of a reference panel is analyzed. The method used is based on the simulation of the directional distribution of diffuse irradiance according to a simplified radiative transfer model (five different atmospheric conditions and 16 sun zenith angles for three different panels). The analysis shows that the relative error for the reflectance factor of a panel can reach 10 % if the diffuse irradiance effect is neglected. A high accuracy simplified method is proposed for correcting for this effect, which takes into account the fraction of diffuse irradiance and its nonisotropic distribution. With this correction, the residual error on the panel reflectance factor is estimated to vary between 0.4 % and 1.0 % in the worst cases.